This invention relates in general to the field of membrane filtration and, more particularly, to an improved device for preventing the bypass of feed flow from the membrane filtration module.
Semipermeable membranes are utilized in both high pressure reverse osmosis and low pressure membrane filtration such as ultrafiltration and microfiltration processes. A typical filtration cartridge within an membrane filtration module includes membranes wound spirally around permeate tubes to form the cartridge. A number of like cartridges can be combined inside of a cylindrical housing, and are typically utilized in series or parallel combinations for filtration of a wide variety of food, pharmaceutical, electrodeposition paint, and industrial waste products.
In applications such as food and pharmaceutical processing, it is essential that the filtration system be thoroughly cleaned after processing the food or pharmaceutical materials. Continual cleaning can also be accomplished during processing by applying an overwrap of plastic netting to the outer diameter of the spiral membrane cartridge to keep the filtering membrane spaced away from its associated housing, and utilizing "controlled bypass" of a small amount of a feed stream along the annular space between the inside wall of the housing and the outer diameter of the filtering membrane cartridge, which space is sometimes referred to as the bypass zone or bypass channel. The flow of fluid within this annular space continually washes away and precludes buildup of material between the cartridge and the housing thereby achieving the goal of thorough cleaning of the bypass zone. The disadvantage of the controlled bypass design is that in most cases a substantial proportion of the feed flow will bypass the filtration membrane, resulting in wasted pump energy and reduced operating efficiency. This occurs because of the practical difficulty in controlling the dimensions of the outer diameter of the cartridge and the inner diameter of the housing to the extent necessary to prevent a loose cartridge fit and preferential feed flow through the resultant gap.
Alternatively, it is sometimes desirable to eliminate the feed flow bypass, as in situations where the fluid being processed tends to accumulate or become lodged in the annulus between the cartridge and the housing. One example of such a situation is the electrodeposition of paint, whereby a paint and water feed must be filtered. It is critical in this application that the paint not stagnate anywhere in the assembly, as it tends to solidify and plug the filtering cartridge, even solidifying the cartridge within the housing, making difficult or even impossible the future removal of the cartridge for cleaning or replacement purposes.
Elimination of this feed flow bypass problem involves sealing the small space or annulus between the outer diameter of the cartridge and the inner diameter of the housing. There are several methods currently available in the industry to seal this space and thereby prevent the bypass problem. One design employs O-ring seals and associated O-ring holders on the opposed ends of the cartridge as an inexpensive, relatively simplistic approach to generating a seal. This technique, however, has several disadvantages. A tight housing tolerance is required, as well as smooth seal surfaces, both of which result in increased housing costs. Also, a large amount of force is required for insertion of large diameter O-rings around the cartridge. Further, the O-ring can sustain damage or even dislodge as the cartridge is positioned within the housing.
The dislodging of or damage to the O-ring, as well as the difficulties encountered in inserting the O-rings, may be circumvented by utilizing the O-ring as a gasket seal. This method, however, is more complicated and therefore more costly, requiring both a large diameter O-ring and an associated compression plate to apply an external force to the O-ring. These two elements result in excessive blockage of the spiral feed channel, which in turn creates an additional pressure drop and decreased filtering performance. The constant compression associated with this compression seal design places the cartridge under constant stress which may result in failure of the spiral module. Further, the compression seal design requires tight dimensional control in an axial direction, resulting in higher cartridge and housing costs.
Another conventional bypass method employs a brine seal and an associated brine seal holder, with the seal installed at one end or both ends of the housing. This design is slightly more expensive than the O-ring design as custom designed seals and molded plastic seal holders are typically required, although it is a desirable bypass method in situations where housing tolerances are too great for O-ring seals to be effectively utilized. Specific installation methods are required for brine seals so as to prevent accidental reverse installation which would render the seals useless. One disadvantage to this design is that the brine seals do not prevent all of the feed flow from leaking through the contact point between the seal and the wall. When the feed stream leaks past the brine seals and into the bypass channel between the cartridge and the inside diameter of the housing, resulting sanitation (e.g., for milk or food processing) and/or cartridge removal (e.g., for electrocoat paint processing) problems tend to occur because of the relatively stagnant flow in this long annular space.
A third conventional bypass method in the industry consists of using a brine seal without an associated holding device. In this method, an elastomeric seal is positioned at one end of the spiral membrane cartridge without the use of a seal holder and is utilized to seal the annular bypass channel. Feed pressure acts to flare open a relatively thin, flexible appendage of the seal forcing it against the housing wall, thereby generating the desired sealing effect. The flexibility and method of operation of this type of seal dictate that these seals only be installed on one end of the cartridge. While brine seals are effective in limiting the bypass flow, the bypass channel behind the seal tends to fill with concentrate resulting in the same sanitation and cartridge removal problems associated with brine seals with holding devices as discussed above. Brine seals without associated holding devices have several other disadvantages, including the loss in productivity which results because the seals are attached to the outside diameter of the spiral rather than to the end of the spiral, severely limiting the membrane area available for processing the fluid. Additionally, if the cartridge is mistakenly installed backwards, the result will be serious bypass which would result in severe membrane performance problems and loss of energy efficiency of the cartridge. Another of the disadvantages of the brine seal is that a housing containing a cartridge utilizing this seal must be mounted in a vertical orientation, as horizontal mounting would automatically result in collapsing the seal under the weight of the cartridge on one side and opening the bypass channel on the opposite side beyond the seal's expansion limits. This eliminates the ability to run modules in series as the height of a vertically oriented system would quickly become a problem. Building parallel instead of in-series configurations to employ brine seal cartridges would greatly increase the system cost, as more valves, fittings, piping and labor would be required. A further disadvantage of the brine seal is that the cartridge using this type of seal cannot be operated in a reverse flow.
A fourth bypass prevention design is polymer potting, a method in which the cartridge is first wrapped in tape, and a liquid material (typically thermoset resin such as epoxy) is then injected or poured into a portion of or the entire annulus between the cartridge and the housing. The liquid material is then allowed to harden and completely seal the bypass channel. While this technique of bypass prevention is very effective, its major limitation is that the cartridge becomes bonded to the housing. As a result, the housing must always be replaced when replacing the cartridge.